Different techniques may be used to dry wood. The most widespread one is that of conventional drying with air-conditioned warm air. Processes under vacuum are also industrially used. One of these consists in using high-frequency vacuum kilns. Batches of wood are disposed in kilns where a partial vacuum is produced to expel the oxygen and reduce the temperatures inside the wood.
The document “Design Considerations of a Commercial Q-Sift Installation for Re-Drying Hemlock Lumber”, Zwick et al., 52nd annual meeting of the Western dry Kiln Association, Reno, Nev., USA, pp. 58-64, May 2001, proposes a process for re-drying wood by means of an equipment made of a conventional industrial batch kiln, a moisture level measuring device, and a high-frequency vacuum kiln (HFV) for re-drying wet pieces. The less strict variability criteria of the final moisture content for the commodity softwoods, produced in very large volume, allow a better possibility of return on investment of a batch HFV kiln for a re-drying application. The sole quality criterion regarding the moisture content for lumber (NLGA standard—National Lumber Grades Authority) is that the moisture content of the dried pieces be under 19% of moisture. The re-drying principle consists in stopping the batch drying cycle before the mean value of the wood moisture content reaches the target value (a few % above this value), carrying the pieces below 19% of moisture to the planer (or other transformation equipment), and re-drying the pieces remaining moist (above 19%) with the high-frequency vacuum. An advantage in doing this additional manipulation is that there are less over-heated pieces. The over-heating causes a greater shrinkage and consequently a more important warping, resulting in a more important potential downgrading of the products. A more important shrinkage imposes a certain oversizing of the sawings at the green state and thus a material yield loss.
The document “The Demonstration of Increased Lumber Value Using Optimized Lumber Sorting and Radio Frequency Vacuum Drying”, Elustondo et al., Forest Prod. J. 55(1): 76-83, 2005, upholds that this technique would be economically advantageous.
However, the re-drying by high-frequency vacuum may prove to be less interesting on an economical point of view than the re-drying by conventional process. Moreover, currently, the industry does not seem convinced of the advantage of batch re-drying (by conventional method or other) since this process involves a lot of additional manipulations (as well as costs). Indeed, batch drying processes require human manipulations when stacking the pieces in the enclosure. Furthermore, the most difficult quality criterion to be obtained with the batch drying processes is a very low moisture variability around the desired target value (for example: it is easy to obtain a mean target of 15% but with some pieces at 7% and other ones at 25% of moisture). To achieve it, it is often necessary to take many additional processing hours inside the kilns.
U.S. Pat. No. 4,258,240 (Pless) proposes a continuous high-frequency process. But the process involves among other things a backward and forward moving of the wood stack, and periodic permutations of the electrode connexions to the generator and to the ground. Furthermore, the process applies to a stack of wood and does not satisfy the need of automatically selecting, controlling and ascertaining the moisture content of each one of the pieces and thus of limiting the necessity to have an operator to do it. The proposed process is rather akin to a semi-continuous application and is not adapted to purely continuous applications involving few or no manipulations.